The present invention relates to an assembly structure of laser diode, and more particularly, to a common assembly structure of vertical cavity surface emitting laser (VCSEL) and monitoring positive-intrinsic-negative (PIN) detector.
In Taiwan, optoelectronic industry is a hi-tech industry next to the electronic industry received intensive focus from the government. Currently, the rapidly developed optoelectronic industry is to combine both electronics and optics so as to explore a new application filed. An optical transmitting and receiving module comprises a transmitter, and a receiver, or the module is integrated as a transceiver.
The function of transmitter is to convert an electronic signal to a light signal and then transmit the light signal. Transmitters are generally classified in accordance with the light source, and the light source of the optical fiber communication is mainly from a light emitting diode (LED) or a laser diode. Since the laser diode has the advantages of high output power, fast transmitting speed, small emitting angle (i.e. a higher efficiency for coupling light source into an optical fiber), and narrower frequency spectrum (smaller dispersion), the laser diode is suitable for use in the transmission of middle or high distance. While LED has the advantages of low cost and simpler utilization (simpler driving and compensation circuits), LED is suitable for use in the transmission of shorter distance. Particularly, the laser diode or called semiconductor laser has the advantages of small size, low power consumption, quick response, good collision resistance, long operation life, and high efficiency, etc., so that the laser diode is very widely used in the application of optoelectronic products. The fabrication of laser diode is delicate and precise, and requires high technical level with the use of expensive manufacturing equipments. Hence, viewing from the whole optoelectronic industry, the laser diode is a very important critical device. According to the wavelength and the applications, the laser diode is roughly divided into a short-wavelength laser and a long-wavelength laser, wherein the short-wavelength laser generally stands for the laser with 390 nm to 950 nm in wavelength, which is mainly used in the applications of optical information and display, for example, CD-ROM drives, laser printers, bar code readers, and indicators, etc., and the long-wavelength laser stands for the laser with 980 nm to 1550 nm in wavelength, which is mainly used in the optical fiber communication. Another rapidly developed laser diode is a VCSEL, which is mainly different from a conventional laser diode in the relative position between a resonant cavity and an epilayer. The conventional diode has the resonant cavity parallel to the epilayer, and a reflecting surface formed by using a naturally cleaved facet normal to the epilayer thereby emitting the laser from the edge, so that the conventional diode is called an edge-emitting laser (EEL), such as a Fabry Perot Laser (F-P Laser) and a distributed feedback laser, etc., while the laser diode rapidly developed has a cavity perpendicular to the epilayer, and a reflecting surface formed with an epilayer or a surface dielectric film thereby emitting the laser from the front, so that the laser diode rapidly developed is called a VCSEL. Since the VCSEL has a very short resonant cavity and few gain media, the output power is extremely limited, and a typical VCSEL has only about few mWs output power. However, the light beam output from VCSEL is circular and symmetrical, which is very easy to couple with an optical fiber and is convenient for forming a VCSEL array, so that VCSEL is expected to replace part of the low-powered EEL diodes for further lowering the system price with a very promising future. Furthermore, the VCSEL with 850 nm in wavelength had been put into the market in 1997, and began to enter various LAN applications since 1998, wherein the VCSEL has been adopted in the standard of gigabit transmission modules, for example, fibre channel link, and gigabit Ethernet, etc., for transmitting a large amount of information within short distance.
Further, the main function of optical receiver is to convert a light signal to an electronic signal, of which the most critical component is a detector. The detector must have the characteristics of low noise, small error, high reliability, high response speed and high sensitivity. The major principle of the detector is to generate enough energy by radiating light on a photo diode for exciting pairs of electrons and holes especially in the depletion region on the pn junction, so as to generate a current signal while a reverse bias is applied externally. The most popularly detectors used in the current optical fiber communication are positive-intrinsic-negative (PIN) detector and avalanche photodiode (APD). The PIN detector is mainly to implement an intrinsic layer between the p-type diode and the n-type diode in a common pn diode to broaden the depletion region for enhancing the current excited from a light source thereby increasing the sensitivity. However, broadening the intrinsic layer also slows down the response speed, so that the balance between increasing sensitivity and maintaining response speed has become the major design consideration. Since a PIN detector is low in cost, and also is good enough for meeting the needs of communication, it is more popularly used. The APD photodiode is mainly different from the PIN detector in producing a higher potential inside the diode and broadening the depletion region to use the electrons and holes excited by the light source as major carriers, and producing secondary carriers by the collisions of the major carriers driven by the high kinetic energy from the high potential, so that a current larger than that from the PIN detector can be generated, wherein the response speed is quicker with the bigger reverse bias. Although the APD photodiode frequently can generate a larger current up to hundreds volts, yet a dark current is formed with the bigger reverse bias, and the dark current becomes one of the noise sources. The so-called dark current means that, due to the high reverse bias, a current can be generated even without the electrons and holes excited by the light source.
A conventional common assembly structure of VCSEL and monitoring PIN detector is formed by connecting an electrode of a VCSEL to an electrode of a PIN detector, such as a common anode assembly of laser diode, and a common cathode assembly of laser diode. FIG. 1a is a top view showing a conventional common assembly structure of a VCSEL and a monitoring PIN detector, and FIG. 1b is a cross-sectional view showing the conventional common assembly structure of a VCSEL and a monitoring PIN detector. Referring to FIG. 1a and FIG. 1b at the same time, the conventional common assembly structure of a laser diode and a PIN detector comprises a PIN detector 10, wherein an electrode region 12 is located on the surface of the PIN detector 10, and a laser diode 20 is in the electrode region 12 on the PIN detector 10. The laser diode 20 has an anode and a cathode, and these electrodes can be located respectively on the upper and lower sides of the laser diode 20, wherein an electrode 22 is on the upper side of the laser diode 20, and an electrode 24 having the different electrical property from the electrode 22 of the laser diode 20 is connected to the electrode region 12 of the PIN detector 10. Furthermore, the common anode assembly of laser diode is to connect the anode of a semiconductor laser and the cathode of a PIN detector to a common electrode, and the common cathode assembly of laser diode is to connect the cathode of a semiconductor laser and the anode of a PIN detector to a common electrode. Since the conventional assembling method is to make the laser diode and the PIN detector use a common electrode, the device assembled has a higher parasitic capacitance and bigger driving load and thus is difficult to be driven with high speed.
In order to improve the aforementioned disadvantage of the conventional technique, the present invention hence provides a common assembly structure of VCSEL and monitoring PIN detector for suitable use in high-speed laser driving circuits. In the assembly structure of the present invention, the anode and cathode of semiconductor laser are completely isolated from the anode and cathode of PIN detector, so as to effectively lower the parasitic capacitance effect resulted from the PIN detector itself and the assembly structure.
Here only VCSEL is explained herein with figures, however the aforementioned description of the present invention is suitable for use in VCSEL, EEL, DFB laser and other semiconductor lasers.
According to the aforementioned object, the present invention provides a common assembly structure of laser diode and PIN detector, and the common assembly comprises: a sub-assembly structure composed of a laser diode formed on a PIN detector without directly contacting the anode and cathode of the PIN detector, wherein the sub-assembly structure comprises: the laser diode formed on a submount with one electrode of the laser diode connected to the conductive region of the submount and the other electrode connected to the electrode of the submount.
Furthermore, the common assembly structure of laser diode and PIN detector according to the present invention can be derived to a structure comprising: a laser diode having an anode and a cathode and being implemented on a submount which is located on a PIN detector having an anode and a cathode, wherein, first, the anode of the laser diode is electrically isolated from the cathode of the PIN detector, and the cathode of the laser diode is also electrically isolated from the anode of the PIN detector, and secondly, the anode of the laser diode and the cathode of the PIN detector are in electrical contact but located in parallel on a low-capacitance submount, so that the equivalent circuit at any terminal is effected by the low-capacitance submount, thereby lowering the RC time constant and promoting the product speed, and thirdly, the anode of the laser diode and the cathode of the PIN detector electrode are in electrical contact but located in parallel on a low-capacitance submount, so that the equivalent circuit at any terminal is effected by the low-capacitance submount, thereby lowering the RC time constant and promoting the product speed.
The common assembly structure of laser diode and PIN detector according to the present invention further comprises a laser diode having an anode and a cathode, and a PIN detector having an anode and a cathode, wherein the anode of the laser diode and the cathode of the PIN detector are electrically isolated, and the cathode of the laser diode and the anode of the PIN detector are also electrically isolated.
In the common assembly structure of laser diode and PIN detector according to the present invention, the submount is made of the low capacitance materials, for example, aluminum nitride, aluminum oxide, glass, quartz and silicon oxide, etc. The common assembling structure of the present invention can also be utilized for the laser diodes such as VCSEL EEL and DFB laser, etc.